A better managment in selection and context

We had this problem when we were editing an element deep down in the model hierarchy. For example, when we are editing a behavior, the current selection is the behavior, and the selection knows nothing about its parent, or the scene in which it is. We have a partial solution with the context, but that wasn't enough.

I created a class called "Selection", who manages selection in a hierarchical way. So, for example, now, when we select a behavior, we'd have the following selection stack:
```
+ scene: "Scene 1"
+ editedGroup: "Scene 1"
+ sceneElement: "A model entity in the scene"
+ behavior: A behavior <--- active context
```
so through the selection object, we can access any of these elements in the hierarchy. For example, with this selection, we press "Add Image", we can check the "editedGroup" to see where the new child must be added; if we press "Copy", we copy the active context, and so on.